Earthquake Warning Systems (EWS) rely on rapid detection and characterization of earthquake ground motions to provide alerts in advance of hazardous shaking. Because the warning time afforded by EWS is short (on the order of a few seconds) there are very few actions that may be taken that have not been premeditated in advance of the event. These actions generally fall into two categories: automated, meaning electronically or electromechanically actuated in response to an electronic signal; or human-mediated, i.e. actuated as the reaction of a human receiving an auditory or visual signal. In general the auditory/visual signal in the latter category is itself an automated action initiated in response to an electronic warning.
Existing EWS typically issue a binary warning (“hazard” or “no hazard”). However, because the cost of taking any action is not zero, it is important to consider, for each individual asset and each separate action, the potential benefit to taking action for a given level of anticipated ground motion (intensity) and a given estimated uncertainty in that level. This analysis can be performed at any desired level of sophistication, but it requires that the electronic warning issued by the EWS encode both the level of anticipated ground motion and the estimated uncertainty in that level. Because time is of the essence, this information must be communicated by the EWS in as compact a form as possible.
The response to an EWS warning can encompass both human-mediated and automated actions. Automated actions must be based on a predetermined policy encoding what to do, and under what warning conditions. In the simplest policy, the only condition is whether a binary warning exists, and the action to undertake does not vary from warning to warning. A more sophisticated policy may encode many different actions, to be undertaken under varying conditions of estimated ground motion and confidence levels. In such a sophisticated system, even human-mediated responses are ultimately automated in that a decision must be made in advance as to what warning conditions trigger notification of the mediating persons.
There is usually a cost with taking an action in response to an earthquake warning. This cost might be the productivity impact of personnel responding to an alert or the lost availability of equipment that is shut down. There may be a significant restart cost for equipment or processes that have been shut down. There is also a cost associated with taking no action to protect people and assets. In many cases, the decision to initiate a response to the earthquake warning is a balance between the expected cost of damage and the cost of a shutdown.
Because different users will have differing requirements for actions to undertake, and conditions under which to initiate those actions, a sophisticated policy engine is needed to encode the user's desired policies and to implement them in the event of a warning from the EWS.